1. Field of the Invention
For communications purposes, data links that use a high data rate (“High-Speed data paths”) are preferably configured using differential signaling. The expression “differential signaling” in this case refers to two signal conductors being provided for the transmission of the data signals and are operated with opposite polarity or in antiphase. The standards TIA-LVDS and EIA-LVDS (LVDS: low voltage differential signals/signaling) exist for standardization of differential signaling.
Differential signaling allows smaller signal amplitudes and is less susceptible to disturbances than signal-phase (signal-ended) signaling in which only a single conductor is used for the transmission of a data signal. Single-phase (signal-ended) signaling is particularly suitable when there are a very large number of signals or the signal density is very high, but can be used only to a very restricted extent for the transmission of high bit-rate signals. Owing to the fact that the data rates are rising all the time, systems based on differential signaling are thus becoming increasingly widely used. Even complete bus systems are nowadays configured to use differential signaling.
As the use of differential signaling increases in modern electronic systems, there is an increasing requirement for the capability to electronically test these systems during manufacture before delivery, and in the field. One particularly major aspect in this case is the testing of the signal connections between the individual assemblies and integrated circuits in a system. Even during manufacture, there is an aim to identify at an early stage in particular contact faults, such as short circuits between signal conductors or to supply conductors, and unconnected (open) signal conductors in order to make it possible to rework these before further system integration, and to replace damaged assemblies.
An electronic testing method for the identification of contact faults has become increasingly widely used in recent years, in which all the inputs and outputs of the components in the system are connected to form a serial sampling register, referred to as a boundary scan register.
This method has been standardized by the IEEE Computer Society and has been published under the title “IEEE Standard Test Access Port and Boundary-Scan Architecture” (IEEE Std. 1149.1). The test architecture specified by the IEEE Computer Society is, however, suitable only for the evaluation of single-phase signals and the identification of the contact faults that may possibly occur on the signal conductors with single-phase signals. However, it is not directly possible to use the cited IEEE Standard with the aim of identifying all possible contact faults when using differential signaling.